A successor communication system to W-CDMA and HSDPA (collectively called UMTS), i.e., Long Term Evolution (LTE), is currently being discussed by 3GPP that is a standardization group for UMTS. In LTE, orthogonal frequency division multiplexing (OFDM) is to be used as a downlink radio access method and single-carrier frequency division multiple access (SC-FDMA) is to be used as an uplink radio access method (see, for example, 3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA,” June 2006).
In LTE, the maximum transmission rate of 100 Mbps is to be supported for downlink. Also in LTE, the transmission rate is optimized for respective users moving at low speed and high speed.
Meanwhile, MIMO transmission (MIMO multiplexing), where different signals are transmitted in parallel via transmission paths formed by multiple inputs (transmitting antennas) and multiple outputs (receiving antennas), is expected to be an indispensable technology for LTE. MIMO transmission makes it possible to increase the total transmission rate by the number of parallel transmission paths even if the frequency band is unchanged.
As a radio access method for high-speed transmission at several tens of Mbps or higher, orthogonal frequency division multiplexing (OFDM) is suitable. In OFDM, orthogonal subcarriers are densely arranged such that the spectra of the subcarriers overlap each other to improve frequency efficiency. In OFDM, a signal is divided and is transmitted via multiple subcarriers. Compared with a method where a signal is transmitted via one carrier, in a transmission method using n (n is an integer greater than 0) subcarriers, the symbol length becomes n times greater.
For example, a transmission method as shown in FIG. 1 has been proposed. In the exemplary transmission method of FIG. 1, a base station (eNode B: eNB) equipped with four antennas transmits shared data channels (SDCH) using the four antennas and transmits L1/L2 control channels using two of the four antennas. Also, the base station transmits reference signals (RS) unique to the respective antennas from the corresponding antennas. A reference signal includes bits that are known to both the sending end and the receiving end before transmission and may also be called a known signal, a pilot signal, and a training signal.
Also, in another proposal, the reference signals corresponding to four antennas of the base station are mapped to leading OFDM symbols in each transmission slot as shown in FIG. 2 (see, for example, 3GPP TS 36.211 (V0.3.0), January 2007).